Add a comment for the ARM_F{0..7}_REGNUM registers

These are for the obsolete FPA architecture.

gdb/ChangeLog:

2020-02-11  Christian Biesinger  <cbiesinger@google.com>

	* arch/arm.h (enum gdb_regnum): Add comment for the FP0..7
	registers.

Change-Id: I6920616318ee637493d4ca12b91fa2ebcd103d76